The polymerization of propylene over Ziegler-type catalysts to produce isotactic propylene polymers is well known in the art. The stereoregularity of such isotactic polymers is typically measured in terms of the percent of the polymer fluff which is soluble in xylene. In general, relative low xylene solubles content polymers are preferred.
Ziegler-type catalysts incorporate a transition metal such as titanium, zirconium or hafnium, which functions to provide sites for the insertion of monomer units into growing polymer chains. One type of such polymerization catalysts involves the so-called homogeneous catalyst systems in which the transition metal compound is a metallocene comprising one or more substituted or unsubstituted cyclopentadienyl groups coordinated with the transition metal atom forming the situs for polymer growth. Such metallocene-based catalyst systems are disclosed U.S. Pat. No. 4,794,096 to Ewen and U.S. Pat. No. 4,892,851 to Ewen et al., for use in the polymerization of propylene to form isotactic or syndiotactic polypropylene.
The more widely used transition metal catalysts are the so-called heterogeneous catalyst systems in which transition metal halides, zirconium, hafnium or titanium, di-, tri-, or tetra-halides, are incorporated with a support structure, principally based upon magnesium or zinc halides, ethoxides or the like. For example, U.S. Pat. No. 4,476,289 to Mayr et al. discloses so-called “activated” titanium tetrahalides, more specifically, titanium tetrachloride, supported on anhydrous magnesium or zinc halides, principally magnesium chloride or magnesium bromide. The transition metal component is used in conjunction with a second component, commonly referred to as a co-catalyst, which as described in the Mayr et al. patent, is a hydride or organometallic compound based primarily upon aluminum, although lithium or magnesium based compounds are also disclosed. A supported catalyst containing yet another component is disclosed in U.S. Pat. No. 4,636,486 to Mayr et al. Here, the titanium, compound, which may be a halide, an oxyhalide or an alcoholate in either the di-, tri-, or tetravalent form, is composited with the magnesium support, together with an electron donor compound. Such electron donors, commonly referred to as internal electron donors because they are incorporated as part of the transition metal catalyst component, can be selected from a broad class of compounds including amines, amides, phosphines, ethers, thioethers, alcohol esters, aldehydes, and ketones. As in the case of the aforementioned U.S. Pat. No. 4,476,289 to Mayr, the catalyst system here also includes a co-catalyst such as trimethy aluminum (TMA) or triethylaluminum, (TEAL).
Yet a third component often employed in Ziegler-type catalyst systems is a so-called external electron donor. The external electron donors function similarly as the internal electron donors and in a complimentary or supplementary manner to regulate monomer insertion into the polymer chain growing on the transition metal active sites. Thus, the electron donors can have an impact upon catalyst activity, polymer molecular weight, and polymer morphology as reflected in stereospecificity and physical parameters such as melting point. For example, in the polymerization of propylene, the addition of electron donors under controlled conditions can result in dramatic increases in activity (the amount of polymer produced per unit of catalyst) and in stereoregularity, e.g., an increase in isotactic polymer content with a corresponding decrease in atactic polymer content.
The complimentary nature of the internal and external electron donors is addressed in Soga, K. et al., “Effect of Diesters and Organosilicon Compounds on the Stability and Stereospecificity of Ziegler-Natta Catalysts”, Transition Metal Catalyzed Polymerizations: Ziegler-Natta and Metathesis Polymerizations, Quirk, R. P., Ed., Cambridge University Press, New York, 1988, pp. 266–279. As discussed in Soga, the concentrations of the internal and external donors in the catalyst system can be adjusted in order to optimize the activity and the stereospecificity of the catalyst. In the experimental work reported there, the transition metal catalyst component comprising titanium tetrachloride supported on magnesium dichloride with an internal donor, e.g., di-N butylphthalate, was slurried in hexane followed by the addition of an external electron donor, phenyl tri-ethoxysilane, and triethylaluminum (TEA) co-catalyst. Soga et al. report on polymerization rates over periods of several hours and isotactic indices measured over periods of several hours for various internal, external catalyst systems at varying concentrations expressed in terms of silicon/titanium mol ratios and TEA/titanium mol ratios.
U.S. Pat. No. 4,287,328 to Kikuta et al., is directed to the polymerization of alpha olefins in the presence of multi-component catalyst systems involving a “solid product” combined with an organoaluminum compound including, for example, C1–C10 trialkylaluminum, triethylaluminum, alkyl alkyoxyaluminums, and alkylaluminum halides, and an electron donor including various organic acids, alcohols, ethers, aldehydes, ketones, amines, alkenol amines, esters, phosphines, phosphites, thioethers, thioalcohols, silanes, and siloxanes. The “solid product” catalyst component is formed by reacting a trivalent metal halide such as aluminum trichloride, aluminum tribromide or ferric trichloride with a divalent metal compound such as magnesium, calcium, or zinc hydroxide or oxide or carbonate with titanium tetrachloride, characterized as an electron acceptor. Numerous orders of additions of the various component are described in Kikuta et al., especially in columns 6 through 9. Conditions of mixing can vary over wide temperature ranges and time intervals, but temperatures are preferably in the range of room temperature to about 100° C. The mixing of the various components can be carried out over periods of several minutes to several hours.
U.S. Pat. No. 4,567,155 to Tovrog et al., discloses multi-component catalyst systems useful in the gas phase polymerization of alpha olefins. In Tovrog et al., the catalyst systems comprise two base catalyst components, each containing subcomponents. The first component, identified as component “A” comprises a titanium component supported on a hydrocarbon insoluble magnesium component in combination with an electron compound. The second major component is a co-catalyst component, characterized as component “B” comprising a trialkylaluminum, an aromatic acid ester and an unhindered secondary amine. Tovrog discloses that the catalyst components may be mechanically activated by comminution prior to use in polymerization. Comminuted catalysts may be pre-polymerized with an alpha olefin before use as a polymerization catalyst component. In the pre-polymerization procedure, comminuted catalysts and an organoaluminum compound co-catalyst are contacted with an alpha olefin under polymerization conditions and preferably in the presence of a modifier such as methyt-p-toluate and an inert hydrocarbon such as hexane, with typical time durations for prepolymerization and other pretreatment procedures involving periods of minutes up to a few hours.
U.S. Pat. No. 4,767,735 to Ewen et al. discloses a pre-polymerization process carried out over a period of less than a minute and usually ten seconds or less. In the Ewen et al. procedure, an organic solvent stream such as hexane or heptane is established in a pre-mixing line. To this stream are added sequentially a co-catalyst (TEAL), an external electron donor (diphenyldimethyoxysilane) and a supported catalyst component (titanium tetrachloride supported on magnesium dichloride) to form a catalyst system which is then pre-polymerized by contact with propylene for a few seconds. An alternative mode of addition in the Ewen et al. procedure is to add the electron donor to the carrier stream after the addition of the titanium catalyst component, but still before the addition of the propylene. Ewen et al. disclose that the cocatalyst should be present when the electron donor and the transition metal catalyst component contact one another in order to avoid poisoning of the titanium catalyst.
High efficiency catalyst systems employing external electron donors which may be characterized generally as sec or tert alkyl or cycloalkyl, alkyl dialkoxy silanes in combination with titanium tetrachloride supported on magnesium based supports derived from dialkoxy magnesium compounds are disclosed in U.S. Pat. No. 4,927,797 to Ewen. By way of example, the supported catalyst may be formulated through the reaction of diethoxy magnesium, titanium tetrachloride, and butyl phthalate under appropriate conditions as specified in the patent. A suitable external electron donor here is methylcyclohexyldimethoxysilane which is compared with diphenyldimethoxysilane as disclosed in the aforementioned Ewen et al. patent.
U.S. Pat. No. 5,432,139 to Shamshoum, et al. discloses the formulation of titanium tetrachloride and other transition metal chloride supported catalyst systems by means of various orders of addition of the catalyst components in order to achieve various results in terms of melt flow indices, polymer bulk density catalyst activities and xylene solubles content. As disclosed in the Shamshoum, et al. patent, various factors such as order of addition and the time in which various components are contacted before the addition of another component can be regulated to arrive at a catalyst having desired characteristics in terms of factors such as activities and bulk density and stereoregularity of the polymer product. Another consideration in the polymerization of propylene is the melt flow index of the polymer product.